A novel type-II–II heterojunction for photocatalytic degradation of LEV based on the built-in electric field: carrier transfer mechanism and DFT calculation

Heterojunctions play a crucial role in improving the absorption of visible light and performance of photocatalysts for organic contaminants degradation in water. In this work, a novel type-II–II Ag2CO3/Bi2WO6 (AB) heterojunction was synthesized by hydrothermal reaction and in situ-precipitation methods. The mechanisms of charge transfer and carrier separation at the interface of heterojunctions and the influence on the photocatalytic activity were investigated. The degradation of levofloxacin (LEV) under visible light irradiation was employed to evaluate the photocatalytic performance of AB. The results showed that 85.4% LEV was degraded by AB, which was 1.38 and 1.39 times higher than that of Bi2WO6 and Ag2CO3, respectively. The work functions of the different crystal planes in the AB heterojunction, which was calculated by density functional theory, are a significant difference. The Fermi energy (Ef) of Ag2CO3 (− 6.005 eV) is lower than Bi2WO6 (− 3.659 eV), but the conduction band (CB) is higher. Therefore, using AB heterojunctions as an example, the research explored the mechanism of type-II–II which CB and Ef of one semiconductor cannot simultaneously surpass those of another material, based on the built-in electric field theory. Through this analysis, a deeper understanding of type-II heterojunctions was achieved, and providing valuable insights into the behavior of this specific heterojunction system.

stability, Bi 2 WO 6 is considered a promising semiconductor photocatalyst for use in the field of photocatalytic degradation [8][9][10][11] .However, the photocatalytic performance of Bi 2 WO 6 is greatly hindered by the high recombination rate of the photogenerated carriers during the transfer process.Various approaches have been explored to enhance the performance of photocatalysts, including ion doping, noble metals-loading, morphology modulation, and heterojunction assembly [12][13][14][15] .Notably, the construction of heterojunction has emerged as an effective strategy to solve the problem of rapid carrier recombination while expanding the range of visible light absorption.Feng Wei et al. synthesized BiOI/Bi 2 WO 6 nanocomposites, which achieved a 99% degradation of methylene blue (MB) under simulated visible light irradiation, representing a 30% improvement over Bi 2 WO 6 16 .Similarly, Yang Jun et al. successfully prepared Bi 2 WO 6 /BiOCl heterojunctions using a one-step hydrothermal method, and reached a 93.3% degradation rhodamine B, a 33% enhancement compared to Bi 2 WO 6 17 .These studies demonstrate that the construction of heterojunction structures can significantly enhance the photocatalytic activity of Bi 2 WO 6 .
Ag 2 CO 3 , known for its good visible light absorption and photocatalytic activity, is also a noteworthy photocatalyst that has attracted substantial attention 18 .In order to overcome the shortcomings of photo-corrosion and electron-hole pair recombination, researchers have dedicated considerable efforts to investigating the construction of heterostructures involving Ag 2 CO 3 , such as Ag 2 CO 3 /TiO 2 19 , Ag 2 CO 3 /Ag/AgBr 20 , WO 3 /Ag 2 CO 3 21 , Ag 2 CO 3 /SnFe 2 O 4 22 , Ag 2 CO 3 /CeO 2 23 , Ag 2 CO 3 /BiOBr/CdS 24 , among others.These studies have paved the way for novel heterostructure designs aimed at enhancing the stability and photocatalytic performance of Ag 2 CO 3 .
To elucidate the impact of heterojunctions on photocatalytic activity, scientists have categorized them into three types: straddling (type-I), staggered (type-II), and gap-breaking (type-III), based on the relative positions of the energy bands of the materials [25][26][27] .About the conventional explanation of type-II heterojunction, e − tends to accumulate in the photocatalyst with a more positive conduction band (CB) potential, while h + transfers to the material with more negative valence band (VB) potential, and enhances light absorption and facilitates charge separation.However, from the kinetic point of view, there is a repulsive force between e − and between h + , posing challenges for the migration of charges in the heterojunction theory.In recent years, with the rapid development in the field of photocatalysis, the role of the built-in electric field (IEF) in the transfer of photogenerated carriers has been recognized [28][29][30][31] .IEF is mainly determined by the Fermi level (E f ) of the two materials forming the heterojunction.Considering the distribution of the CB and the E f , the type II heterojunction can be classified into two types: (i) CB and E f of one semiconductor are higher than those of another, it is named type-II-I in the article; (ii) CB and E f of one semiconductor cannot simultaneously surpass those of another material, it is named type-II-II.Figure 1 is detailed interpretation of the above-mentioned.Regarding the carrier transfer mechanism of type-II-I heterojunction, scholars have offered an explanation based on the IEF theory 32 .However, the photocatalytic mechanism of type-II-II heterojunction remains unexplored.Therefore, we focused on type-II-II heterojunctions and used Ag 2 CO 3 and Bi 2 WO 6 to prepare it, where the CB and E f of Ag 2 CO 3 are not higher than those of Bi 2 WO 6 simultaneously.By investigating the carrier transfer process, we aim to shed light on the influence of the E f on the IEF of the heterojunction in the type-II-II heterojunction.
In this work, Ag 2 CO 3 /Bi 2 WO 6 (AB) nanocomposites were prepared by hydrothermal and in situ precipitation methods.The structure and properties of the materials were characterized and analyzed.The photocatalytic degradation performance of the nanocomposites towards levofloxacin (LEV) was investigated to evaluate their activity, stability, and applicability.The transfer mechanism of photogenerated carriers in AB heterojunctions was

Preparation of photocatalysts
Bi 2 WO 6 nanosheets were prepared by a hydrothermal method.Initially, 0.97 g of Bi(NO 3 ) 3 ‧5H 2 O was dispersed in 30 mL of 0.5 mol/L nitric acid solution and stirred for 1 h until completely dissolved.Subsequently, 0.33 g Na 2 WO 4 ‧2H 2 O was added into 30 mL of deionized water, completely dissolved, and then added dropwise into Bi(NO 3 ) 3 solution, stirred for 1 h.The resulting suspension was transferred into 80 mL of Teflon-lined stainless autoclave and heated at 160 °C for 18 h.Afterward, the obtained product was washed thrice with deionized water, and the yellow precipitate was dried in an oven at 60 °C for 12 h.
A in situ-precipitation method was used to fabricate AB nanocomposites.Specifically, 2 g of Bi 2 WO 6 nanoflakes were dispersed in 20 mL of deionized water, and subjected to ultrasonication for 30 min.Subsequently, 14.3 mL of AgNO 3 solution (0.10 M) was added to the above suspension and stirred for 30 min under dark conditions to promote the adsorption of silver ions on the surface of Bi 2 WO 6 .Then, an equal volume of Na 2 CO 3 solution (0.05 M) was added and the mixture was stirred for 4 h under dark conditions.The resulting sample was washed three times, filtered, and then dried at 60 °C for 12 h.Once sufficiently dried, the sample was ground and bagged.

Photocatalyst characterization
The crystal structure and surface morphology of the photocatalysts were characterized and analyzed by X-ray powder diffraction (XRD) (China, Puxi General Instrument, XD3) and field emission scanning electron microscopy (SEM) (Germany, ZEISS, Sigma 300).An ultraviolet-visible spectrophotometer (UV-Vis) (Japan, Shimadzu, UV-360Oi Plus) was used to analyze the photo response ability of the samples with a scanning range of 200-800 nm.X-ray photoelectron spectroscopy (XPS) (USA, Thermo Scientific, K-Alpha) was used to examine the surface composition, the valence information, and the surface chemical bonding states of the samples.In addition, the separation efficiency of carriers was evaluated by a fluorescence spectrometer (China, Gangdong Sci &Tech F-380).A 3-electrode electrochemical analysis approach was used for electrochemical impedance spectroscopy (EIS) using an electrochemical workstation (China, Chenhua Instrumen, CHI 660E).The intermediates produced during the process of degradation of LEV were measured and analyzed by liquid chromatographytandem mass spectrometry (LC-MS) (Japan, Shimadzu, LCMS-8050).The degree of mineralization of LEV was measured by total organic carbon (TOC) (Japan, Shimadzu, TOC-L).

Photocatalytic degradation experiment
To evaluate the photocatalytic performance of AB nanocomposites, the degradation of LEV was conducted in a temperature-controlled photochemical reactor (NAI-GHY-DSGKW) equipped with multiple tubes.A 500 W xenon lamp (illuminance: 1369 W/m 2 ) was employed to simulate the visible light irradiation.Before photocatalytic degradation, 0.05 g of photocatalyst was added into 50 mL solution of LEV (10 mg/L), and stirred under dark conditions for 60 min to reach the adsorption-desorption equilibrium.Then, the light source was turned on to carry out the photocatalytic reaction.At certain time intervals, a 3.0 mL sample was withdrawn from the photoreactor and filtered through a 0.45 µm membrane.The concentration of LEV was determined using UV-vis at a wavelength of 288 nm.To fit the LEV degradation data and calculate the rate constant (k) of LEV degradation, a pseudo-primary kinetic model was employed.In addition, IPA, BQ, and EDTA were added as scavengers of •OH, •O 2 − , and h + , respectively, to investigate the contribution of different radicals to the photocatalytic degradation of LEV.The concentration of all three trapping agents were 1.0 mmol/L.•O 2 -and •OH were checked by electron spin resonance (ESR) (Germany, Bruker Corporation, Bruker EMX plus,) with 5,5-dimethyl-1-pyrroline N-oxide (DMPO).

DFT calculations
In this paper, the state density, band structure, and work function of the materials have been calculated using the Materials Studio 2020 (MS) software package based on DFT.The interactions between electrons and ions are described using the projector augmented wave (PAW) method, and the exchange-correlation potential is treated using the PBE generalization in the generalized gradient approximation (GGA).The convergence criteria for the energy and interatomic forces are 10 −5 eV and 0.01 eV/Å, respectively.

Results and discussion
Morphology, structure, and elemental analysis of AB The XRD patterns of Ag 2 CO 3 , Bi 2 WO 6 and AB nanocomposites are shown in Fig. 2. Six diffraction peaks of Ag 2 CO 3 appear at 18.56°, 20.52°, 32.59°, 33.64°, 37.05°, and 39.57°, corresponding to crystal planes (0 2 0), (1 1 0), (− 1 0 1), (1 3 0), (2 0 0) and (0 3 1), respectively.These results are consistent with the known diffraction pattern of Ag 2 CO 3 (PDF#70-2184) 33 .The diffraction peaks of Bi 2 WO 6 (PDF#39-0256) are at 28.3°, 32.67°, 46.9°, and 55.82°, corresponding to (1 3 1), (0 6 0), (2 6 0), and (3 3 1) crystal planes, respectively 34 .Importantly, no diffraction peaks associated with impurities were observed, indicating that the prepared materials have high crystallinity and purity, as shown in Fig. 2a.In Fig. 2b, the XRD pattern of AB nanocomposites reveals primarily the diffraction peaks Bi 2 WO 6 , with the emergence of characteristic peaks of Ag 2 CO 3 as the Ag 2 CO 3 content in the material increased.This observation confirms that AB nanocomposites were successfully synthesized.SEM images of Bi 2 WO 6 , Ag 2 CO 3 , and AB-9 photocatalyst are presented in Fig. 3. Bi 2 WO 6 exhibits nanosheets with smooth surfaces, forming an overall irregular polyhedral morphology with multiple crevices, as shown in Fig. 3a and b.Ag 2 CO 3 , on the other hand, appears as cuboid and micro-spherical particles with smooth surfaces, as illustrated in Fig. 3c and d.The SEM images of the AB-9 reveal a tight bonding between Ag 2 CO 3 and Bi 2 WO 6 , forming a new pore structure, as shown in Fig. 3e and f. Figure 4 displays the energy dispersive spectrometer (EDS) spectrum of the AB-9.The presence of Ag, C, W, Bi, and O, is observed in the spectrum.Furthermore, the elements are uniformly distributed throughout the AB-9, indicating successful preparation of the AB nanocomposite.The morphology of AB-9 was further studied by the TEM, as shown in Fig. 5a.TEM images show that the Bi 2 WO 6 nanosheets are closely covered by Ag 2 CO 3 , which confirms the growth of Ag 2 CO 3 nanoparticles on Bi 2 WO 6 .Figure 5b is the HR-TEM image of Ag 2 CO 3 /Bi 2 WO 6 .the lattice spacing of 0.252 nm corresponds to the (− 2 0 1) crystal plane of Ag 2 CO 3 , and the lattice spacing of 0.207 nm corresponds to the (1 1 2) crystal plane of Bi 2 WO 6 .This is further evidence that the AB heterojunction was successfully constructed.
The chemical composition and elemental valence states of the AB-9 surface were analyzed by the XPS technique.Figure 6a displays the presence of Ag, C, O, Bi, and W elements in the nanocomposite AB-9, consistent with the EDS analysis.The fine spectra of Ag, C, O, Bi, and W elements are shown in Fig. 6b-f, respectively.In Bi 2 WO 6 , the characteristic peaks of Bi 4f. are observed as two strong peaks at 163.98 eV (Bi 4f 7/2 ) and 158.68 eV (Bi 4f 5/2 ), indicating the presence of Bi 3+ ions 35 .The two independent peaks of W 4f. are 37.08 eV (W 4f 5/2 ) and 34.88 eV (W 4f 7/2 ), suggesting the existence of W 6+ ions 36 .Comparatively, in the AB-9 nanocomposite, the binding energies of both Bi 4f. and W 4f. elements are higher than in Bi 2 WO 6 , suggesting that both Bi and W elements in Bi 2 WO 6 lost electrons during the composite process with Ag 2 CO 3 .
In Ag 2 CO 3 , the Ag 3d spectrum exhibits two independent peaks of 373.58 eV (Ag 3d 3/2 , Ag + ) and 367.58 eV (Ag 3d 5/2 , Ag + ) 22,37 .The C 1s spectrum displays two distinct characteristic peaks of 288.38 and 284.18 eV, with the former mainly attributed to the C=O bonding in Ag 2 CO 3 , and the latter to the amorphous carbon 38 .By contrast, the binding energies of both Ag 3d and C 1s in the AB-9 are reduced.The O 1s spectrum of Bi 2 WO 6 exhibits two peaks of 529.48 and 531.08 eV, corresponding to oxygen in the Bi-O and W-O bonds, respectively 39 .In the O 1s spectrum of Ag 2 CO 3 , the characteristic peak at 530.55 eV is attributed to the lattice oxygen, while the characteristic peak at 532.04 eV belongs to the adsorbed oxygen 40 .The increase in the binding energy of O 1s in the AB-9 suggests that the elemental oxygen loses electrons, possibly due to the changes in work function, lattice potential electronegativity difference, and other factors.However, undoubtedly, this phenomenon also confirms the existence of interaction between Bi 2 WO 6 and Ag 2 CO 3 .In general, the binding energy of Bi 2 WO 6 is shifted to the high-energy region, while that of Ag 2 CO 3 is shifted to the low-energy region, indicating that the electron density of Bi 2 WO 6 is lower and that of Ag 2 CO 3 is higher within the AB-9.This also suggests that the electrons are transferred from Bi 2 WO 6 to Ag 2 CO 3 in the AB-9 upon contact between the two monomers.These results unequivocally demonstrate the successful combination of the two materials, and the formation of a heterojunction.AB-9, which has the best photocatalytic degradation effect, was employed as a photocatalyst to investigate the impact of AB dosage on LEV degradation.Figure 7c elucidates the influence of AB-9 dosage on the degradation rate of LEV.The degradation rate of LEV reached 84.55% when the dosage of AB-9 was increased from 0.01 to 0.05 g.However, with further increases in dosage, the degradation rate only exhibited a marginal improvement of 5.25%, reaching 89.8%.Similarly, the corresponding photocatalytic degradation rate constant k, as shown in Fig. 7d, exhibits a six-fold change with the addition of photocatalyst from 0.01 to 0.05 g, and a smaller change from 0.05 to 0.07 g.Although, increasing the photocatalyst dosage provides more active sites for the photocatalytic degradation reaction, excessive amounts of photocatalyst may hinder the transmission of light paths in the solution.This can impede its absorption ability for visible light and hinder the incident light irradiation from reaching the solution's interior.Consequently, the number of photogenerated carriers decreases, thereby affecting the photocatalytic degradation performance of AB-9.
The pH of the solution is the main factor influencing the photocatalytic degradation of organic pollutants.Figure 8a and b shows the photocatalytic effect of AB-9 on LEV and the degradation rate constant k at varying pH.At pH 4.11 and 6.03, the degradation rate of LEV reached 86.72 and 84.48%, respectively, accompanied by the high degradation rate constants k.These results indicate that LEV can be degraded rapidly under environmental pH, showing excellent photocatalytic degradation ability.However, at pH 2.9, the degradation rate of LEV declined to 53%.The decline may be attributed to the fact that e -cannot stabilize under the highly acidic environment, and cannot further react to generate •O 2 − .Moreover, the degradation rate of LEV was 65.56 and 37.51% at pH 9.2 and 11.15, respectively.Under alkaline conditions, h + reacts with hydroxyl to form •OH, which exhibits poor stability and participates in photocatalytic degradation in small amounts.As the alkalinity of the solution increases, the degradation of LEV further decreases, indicating that h + may play a major role in the degradation process.
Wastewater usually contains a diverse array of inorganic anions, and the environment in which CO showed that the addition of SO 4 2− and NO 3 − had minimal influence on the photocatalytic performance of the catalyst.However, the addition of Cl − reduced the photocatalytic ability to 64.22%.It is commonly believed that Cl − readily adsorbs on the catalyst surface, impeding the adsorption of reactant molecules and thereby inhibiting the reaction 41 .Following the addition of CO 3 2− , the photocatalyst achieved a mere 27.16% degradation of LEV.This can be attributed to the reaction of h + with CO 3 2− generated by visible light excitation catalyst, resulting in h + quenching and rendering it ineffective in the photocatalytic degradation reaction.
To further investigate the practical applicability of photocatalyst AB-9, several common pollutants were selected for degradation testing, and the results are shown in Fig. 8d.Under the same conditions employed for the photocatalytic degradation of LEV, AB-9 exerted a better degradation effect towards enrofloxacin (ENR) and ciprofloxacin (CIP), achieving degradation levels of 83.37 and 82.68%, respectively.By contrast, AB-9 exhibited less effectiveness in degrading rhodamine B (RhB) and methylene blue (MB).This disparity can mainly be attributed to the fact that ENR and CIP, being quinolone antibiotics with similar properties and stability as LEV, are more amenable to degradation with AB-9.On the other hand, MB and RhB, as common azo dyes, contain many conjugated structures that required higher energies of reactive radicals for degradation.Therefore, when compared to quinolone antibiotics, the photocatalyst AB-9 displays limited capability in degrading azo dyes.
The stability of the photocatalyst AB-9 was evaluated through the reusability experiment.After five cycles, the degradation activity of photocatalyst AB-9 on LEV exhibited a slight reduction, yet the degradation efficiency was still maintained at approximately 75%, as shown in Fig. 8e.To further explore the stability of the sample, the AB-9 was characterized by SEM (Figure S2) and XPS (Figure S3) before and after the reaction.The morphology and chemical composition of the sample before and after photocatalysis have negligible changes.It is further confirmed that the photocatalyst AB-9 is a relatively stable photocatalytic degradation material.In Table 1, this work is compared with the photocatalyst degradation of LEV reported in other literature.Compared with other degradation work, the photocatalyst prepared in this paper has a better degradation effect on LEV.

Optical properties of nanocomposite AB
To investigate the visible light absorption properties of Bi 2 WO 6 , Ag 2 CO 3 , and nanocomposite AB-9, the UV-vis diffuse reflectance spectroscopy (UV-vis-DRS) was employed for analysis.As shown in Fig. 9a, the absorption threshold of Bi 2 WO 6 is approximately 456 nm, which indicates limited absorption of visible light, and restricts its photocatalytic performance.On the other hand, Ag 2 CO 3 exhibits an absorption edge at around 545 nm, indicating a certain degree of visible light absorption capability.Upon combining the two photocatalytic components to form the AB-9 nanocomposite, the visible light absorption ability of the photocatalyst AB-9 is notably improved.This enhancement can be attributed to the synergistic interaction between the two photocatalysts.Based on the crystal structures of Ag 2 CO 3 and Bi 2 WO 6 obtained by XRD analysis, and combined with DFT calculations, the energy bands of Ag 2 CO 3 and Bi 2 WO 6 were determined to be 0.779 and 2.019 eV, respectively, as shown in Fig. 9c and b.The highest occupied molecular orbital of Ag 2 CO 3 is located at the high-symmetry point G, while the lowest unoccupied molecular orbital is situated at the high-symmetry point B, indicating that Ag 2 CO 3 is an indirect bandgap semiconductor.Conversely, both the highest occupied molecular orbital and the lowest unoccupied molecular orbital of Bi 2 WO 6 are positioned at the high symmetry point G, making it a direct bandgap semiconductor.It is worth noting that the bandgap obtained through GGA-PBE function tends to be smaller than the actual bandgap 48 .Analyzing the density of states of Bi 2 WO 6 (Fig. 9d), it becomes evidently the CB is mainly contributed by O and Bi atoms, while the valence band (VB) originates from O and W atoms. On the other hand, examining the density of states of Ag 2 CO 3 (Fig. 9e), it can be observed that the CB is mainly influenced by O and Ag atoms, while the VB is derived from Ag atoms.
To accurately determine the bandgap of the photocatalyst, the UV-vis-DRS data was processed using that Tauc equation αhυ = A (hυ-E g ) n/2 .Where α is the absorption constant, h is Planck's constant, υ is the optical frequency, A is the proportionality constant, E g is the bandgap, and n is a constant associated with the semiconductor's carrier leaps.For direct semiconductors, n = 1, while for indirect semiconductors, n = 4.The Tauc plots   for Ag 2 CO 3 and Bi 2 WO 6 , are shown in Fig. 10a and b.From these plots, it can be concluded that the band gaps of Ag 2 CO 3 and Bi 2 WO 6 are 2.06 and 2.94 eV, respectively.The valence band X-ray photoelectron spectra (VB-XPS) are presented in Fig. 10c and d, where the VB values of Ag 2 CO 3 and Bi 2 WO 6 are 1.59 and 2.53 eV, respectively.Additionally, the calculation yields the CB values of Ag 2 CO 3 and Bi 2 WO 6 as − 0.47 and − 0.41 eV, respectively.The carrier separation efficiency was investigated using fluorescence spectroscopy, and the results are shown in Fig. 11a 49 .The findings show that the emission peaks of both Bi 2 WO 6 and Ag 2 CO 3 monomers are higher than those of the nanocomposite AB.Notably, the emission peak intensity of Bi 2 WO 6 is the highest, indicating that the recombination of e − and h + pairs within Bi 2 WO 6 occurs more easily.The emission peak intensity of Ag 2 CO 3 is relatively low, possibly due to its nature as an indirect bandgap semiconductor.Comparatively, the nanocomposite AB-9 exhibits a reduced fluorescence intensity in comparison to Ag 2 CO 3 and Bi 2 WO 6 monomers.This suggests that the heterojunction formed between Ag 2 CO 3 and Bi 2 WO 6 significantly mitigates the carrier recombination rate.Subsequently, to further investigate the photoelectron transfer rate, EIS plots of Ag 2 CO 3 , Bi 2 WO 6 and AB-9 were constructed in 0.1 Hz-0.1 M Hz.The equivalent circuit for the system is presented in the inset image of Figure S4.In this model, R 0 , R 2 , W 3 , and C 1 respectively represent the resistance between the fluorine-doped tin oxide and catalysts, the charge migration resistance across the photoanode/electrolyte interface, the Warburg

Photocatalytic mechanism
To elucidate the degradation mechanism of the AB-9 nanocomposite on LEV under visible light, free radical capture experiments were conducted to identify the key free radicals involved in the degradation process.Scavenging of •OH, •O 2 − and h + was performed with 5 mM of IPA, BQ and EDTA, respectively.The results, as seen in Fig. 11b, demonstrate that the degradation rate reached 86.25% when only the photocatalyst was added to the LEV solution.Upon the addition of EDTA, the degradation rate of LEV decreased significantly to 8.49%, indicating a 78% reduction.This confirms the crucial role of h + in the degradation process of LEV.Furthermore, the introduction of BQ resulted in a 67% decrease in the degradation rate of LEV, indicating that •O 2 − serves as a secondary reactive radical in the degradation process.When IPA was added, the degradation rate of LEV was unchanged, indicating that •OH had little influence on the degradation process.In summary, the nanocomposite AB mainly employs h + and •O 2 − as the main contributors to the degradation of LEV.To further explore the photocatalytic reaction mechanism, the ESR spectroscopy was carried out to determine the active species generated from the photocatalytic system with AB-9 as the photocatalyst, as shown in Fig. 11c  and d 51 .Furthermore, the DMPO-•OH spectrum fails to exhibit its characteristic peaks, so the process does not produce •OH radicals.This result is consistent with that of the free radical capture experiment.
Based on the energy band distribution of the AB nanocomposite, it can be assigned to a type-II heterojunction.The conventional interpretation of the photogenerated carrier transfer path for type II heterojunction is shown in Fig. 12a.According to this interpretation, e − in the CB of Ag 2 CO 3 drifts to the CB of Bi 2 WO 6 , while h + in the VB of Bi 2 WO 6 drifts to the VB of Ag 2 CO 3 , thus completing the separation of the photogenerated carriers.However, from the kinetic point of view, the repulsive forces between e − (or between h + ) can hinder the transfer of the aforementioned assumptions.
In recent years, the influence of the IEF on the transfer of photogenerated carriers has been recognized by many researchers.It has been mainly employed to explain the cases involving heterojunctions formed by a semiconductor with both a higher CB and a higher E f compared to another semiconductor, and type-II-I is appropriate for this.For type-II-II heterojunctions such as AB, where the photocatalytic performance can be improved compared to the monomer despite the CB and E f of Ag 2 CO 3 not being simultaneously higher than those of Bi 2 WO 6 , we attempted to provide a complementary analysis of the charge transfer mechanism of type-II-II heterojunctions based on the effect of the IEF on carrier migration.
The work function (Φ) of a material represents the work done for an electron to move from the E f to the vacuum energy level (E V ).DFT calculations reveal that Φ is 6.005 and 3.659 eV for the Ag 2 CO 3 (1 3 0) and Bi 2 WO 6 (1 3 1) facets, respectively, as shown in Fig. 12b and c.The Φ of the Ag 2 CO 3 (1 3 0) facet is higher than the Bi 2 WO 6 (1 3 1) facet, indicating that the E f of Ag 2 CO 3 is lower than that of Bi 2 WO 6 (Fig. 12d).When Ag 2 CO 3 and Bi 2 WO 6 form a heterojunction, e − spontaneously transfers from Bi 2 WO 6 to Ag 2 CO 3 due to the different E f (Fig. 12e).As the E f of the two materials reach equilibrium, an electron depletion layer is formed on the Bi 2 WO 6 side, and an electron accumulation layer is formed on the Ag 2 CO 3 side, resulting in the formation of an IEF pointing from Bi 2 WO 6 to Ag 2 CO 3 .The IEF distribution leads to the recombination of e − on the Ag 2 CO 3 CB and h + on the Bi 2 WO 6 VB, meanwhile retaining the e − on the Bi 2 WO 6 CB and h + on the Ag 2 CO 3 VB, which have a relatively low redox capacity (Fig. 12(f

Possible degradation pathways
To elucidate the photocatalytic degradation process of target pollutants, the intermediate products were further determined by high-performance liquid chromatography mass spectrometer (LC-MS).By analyzing the obtained results and considering previous studies, three possible photodegradation routes for LEV are depicted in Fig. 13 (detailed intermediates molecular formula seen in Table S1 and Figure S7).In pathway I, P1 is formed by piperazine ring cleavage of LEV (m/z = 362), and P2 is formed after dialdehyde group and defluorination.P2 may have been completely removed by decarboxylation or piperazine rings to produce P3 or P4.After quinolone ring cleavage and a series of reactions, P4 generates P5.In Pathway II, P6 is formed by decarboxylation of LEV.Subsequently, the quinolone ring in P6 is cleaved to form P7. Piperazine ring cleavage and demethylation form P8. In Pathway III, LEV is decarboxylated first, and then methyl groups are oxidized to form carboxyl groups, giving P9.After further decarboxylation, P10 is formed.After epoxidation of piperazine and cracking of the morpholine ring, P11 and P12 were formed.Finally, all the intermediates are mineralized into small organic molecules, carbon dioxide, and water after a multi-step oxidation reaction, and the photocatalytic degradation of LEV by AB-9 is achieved.Furthermore, the removal of total organic carbon (TOC) was studied (Figure S6).After the photocatalytic reaction for 60 min, the mineralization rate of the solution can reach 42.5%, which further indicates that the LEV can be finally completely mineralized into carbon dioxide and water.

Conclusion
In this paper, type-II heterojunction is classified into type-II-I and type-II-II considering the relative positions of CB and E f in semiconductors.The transfer mechanism of photogenerated carriers in type-II-II heterojunction was investigated through the photocatalytic degradation of LEV by Ag 2 CO 3 /Bi 2 WO 6 heterojunction.
The results show that the degradation rate of AB-9 to LEV reached 85.4% under the condition of visible light irradiation in 60 min.The degradation mechanism is illuminated by DFT calculation, which confirms the e − in Bi 2 WO 6 transfer to Ag 2 CO 3 at the heterojunction interface because the E f (− 3.659 eV) of Bi 2 WO 6 is higher than that of Ag 2 CO 3 (− 6.005 eV).This calculation result is also demonstrated by the variation of the binding energy of elements in XPS.When the E f is equal at the heterojunction interface, the electron depletion layer and accumulation layer are formed at the interface of Bi 2 WO 6 and Ag 2 CO 3 , respectively, and thus an IEF is built from Bi 2 WO 6 to Ag 2 CO 3 .Under the action of the IEF, e − in the CB of Ag 2 CO 3 is recombined with h + in the VB of Bi 2 WO 6 , meanwhile, h + in the VB of Ag 2 CO 3 and e − in the CB of Bi 2 WO 6 is retained.The results of fluorescence spectra and EIS on the type-II-II heterojunction Ag 2 CO 3 /Bi 2 WO 6 indicate that the separation of photogenerated carriers is significantly improved, which is circumstantial evidence of the formation of the IEF.The degradation ability of Ag 2 CO 3 /Bi 2 WO 6 heterojunction to azo dyes with strong stability is slightly insufficient, which may be caused by the retention of free radicals with the weaker redox ability during the photogenerated carriers migration process in type II-II heterojunctions, and evidence the formation mechanism of the IEF.In general, we propose the classification of type-II heterojunctions and analyze the carrier transfer mechanism of type-II-II heterojunctions, which is an important supplement to the theory of type-II heterojunctions.

Figure 1 .
Figure 1.Conventional type-I, II,and III heterojunctions, carrier transfer paths, and classification of type-II.

Figure 6 .
Figure 6.(a) Full elemental spectra of photocatalysts, (b-f) Fine spectra of Bi, W, Ag, C and O elements.

Figure 7 .
Figure 7. (a, b) Degradation of LEV with different materials and degradation rate constant k, (c, d) Degradation of LEV with different AB-9 dosage and degradation rate constant k.

Figure 8 .
Figure 8. (a, b) Degradation of LEV by AB-9 and degradation rate constants k at different pH, (c) Degradation rate of LEV by AB-9 with different anions, (d) Degradation rate of different pollutants by AB-9 (e) reusability of AB-9.
. It should be noted that no peaks of DMPO-•O 2 − and DMPO-•OH are found according to the test data in the dark.However, upon 5 min of visible light irradiation, strong peak signals with an intensity ratio of 1:1:1:1 for DMPO-•O 2 − were detected, indicating the generation of •O 2 − radicals

Figure 12 .
Figure 12.(a) Conventional illustration of type II Heterogeneous, (b, c) Work functions of Ag 2 CO 3 and Bi 2 WO 6 , (d-f) Formation conditions of the IEF and the carrier transfer in AB.

Table 1 .
Comparison of some photocatalytic systems degrading LEV.

system Antibiotic conc. (10 mg/L) Catalyst dosage (g/L) Irradiation time (min) Photocatalytic efficiency (%) Refs.
50and the constant phase element50.In principle, a smaller EIS radius and R 2 represent the faster the charge migration rate.In the EIS plots, AB-9 manifests a smaller arc radius and the lower R 2 of 3933 Ω compared with Ag 2 CO 3 (R 2 , 4031 Ω) and Bi 2 WO 3 (R 2 , 5203 Ω), indicating the lowest charge-migration resistance and the best charge transfer effect. impedance )).The CB potential of Bi 2WO6 has a more negative value than that of O 2 /•O 2 − (− 0.33 eV), allowing the e − on the CB of Bi 2 WO 6 to easily react with O 2 (ad.) to form •O 2 − .On the other hand, the VB potential of Ag 2 CO 3 is not more positive than •OH/OH − (2.23 eV), preventing the VB of Ag 2 CO 3 from generating •OH and participating in the photocatalytic reaction.Eventually, LEV was effectively degraded by •O 2 − and h + .